Control circuit for cooling fans

ABSTRACT

A control circuit for an electrically driven cooling fan has a relay for switching the fan in and out. Associated with the relay is resistance means for reducing the differential between the on and off points of the fan.

United States Patent, 1191, I

Holt Feb. 26, 1974 CONTROL CIRCUIT FOR COOLING FANS [56] ReferencesCited [75] lnventcr: William David Holt, Colne, England v I UNITEDSTATES PATENTS 3,668,497 6/1972 Freiberger et a1. 1. 318/472 X [73]Asslgnee' g iggl Llmlted 2,976,463 3/1961 Adams 3l8/473 X g g 3,416,06012/1968 Evaldsw 318/473 [22] Filed; p 1972 3,500,074 3/1970 Obenhaus3I8/473 ux [211 Primary Examiner-B. Dobeck Attorney, Agent, orFirm-Holman & Stern [30} Foreign Application Priority Data Dec. 1, 1972Great Britain 1,380/72 [57] ABSTRACT I A control circuit for anelectrically driven cooling fan [52] 318/471 317/13 318/473 has a relayfor switching the fan in and out. Associated [51 Int. Cl. 205d 23/30with the relay is resistance means for reducing the [58] held of Search318/471 3 1 70 1 3 ferential between the on and off points of the fan.

1 Claim, 1 Drawing Figure I IS F )1 1 i 16 L .1

I U I9 17 Cf /|6 28 I3 lag l2 CONTROL CIRCUIT FOR COOLING FANS vtemperature of the apparatus falls. to a predetermined level, andresistance means which is coupled to the control circuit by way of therelay contact when the relay winding is energised, the resistance meansreducing the required increase in the. temperature of the apparatusbefore the relay winding is again de-energised to cause the fan tooperate.

The accompanying drawing is a circuit diagram illustrating one exampleof the invention.

Referring to the drawing, a road vehicle incorporates a battery 9 havingits negative terminal connected to an earth supply line 10, and itspositive terminal connectedto a supply'line 11 through a series circuitincluding the vehicle ignition switch 8, diode 30 and a resistor 13. Theroad vehicle is driven by an internal combustion engine indicated'at 17,the engine being cooled by flow cooling water within a conduit 16.

Located within part of the conduit 16 is atemperature-sensitive-resistor one end of which is connected to the line10, and the otherend of which is connected through a temperature gauge14 to the line 11. A junction of the gauge 14 and resistor 15 isconnected through a resistor 24 to the line 10, and a variable point onthe resistor 24 isconnected to the base of an n-p-n transistor 25, theemitter of which is connected to the base of an n-p-n transistor 26having its emitter connected to the line 10. The transistors 25, 26 havetheir collectors connected to the line 11 through a relay winding 22bridged by a freewheel diode 31, and the winding 22 serves when it isenergised to move a contact 23 out of engagement with a contact 21 intoengagement with a contact 28. The contact 28 is connected through avariable resistor 29 to the base of the transistor 25, and the contact21 is connected to the positive battery terminal through a fan motor 19,which when it is operating drives a fan 18 providing cooling air througha part 16a of the conduit 16.

In operation, when the engine temperature is low, the resistor 15 has ahigh value, and current flowing duct sufficiently to energise thewinding 22, so that the contact 23 is in engagement with the contact 21and the fan motor 19 is energised.

Assuming that the fan motor 19 is operating then as the temperature ofthe resistor 15 falls, an increase in base current flows to thetransistor 25, until at a predetermined temperature, the transistors 25,26 conduct sufficiently to energise the winding 22, thereby moving thecontact 23 back into engagement with the contact 20a and stopping themotor 19. However, the contact 23 now couples'the base of the transistorto the line 10 by way of the resistor 29, so reducing the base currentof the transistor 25, and reduces the current flowing in the winding 22.This reduction is insufficient to de-energisethe winding 22, so that thecontact 23 remains in engagement with the contact 28.

As the fan motor 19 is notnow operating, the temperature of the'resistor15 increases, and the current flowing to the winding 22 decreases, untilat a predetermined temperature the winding 22 is de-energised, and thecontact 23 moves into-engagement with the contact 21 again to operatethe motor 19.

It will be appreciated that the relay 22, 23 has the through the gauge14 turns on the transistors 25, 26

the winding 22 is greater than the current at which the winding 22becomes de-energised. In effect, the resistor 29 serves when the winding22 is energised to de crease the current flowing through the winding 22by an amount less than the differential between the turnon and turn-offcurrents of the winding 22.

The diodes 30, 31 are incorporated to protect the transistors 25, 26against transients. The settable resistors 24, 29 determine theoperating points of the fan I claim:

1. A control circuit for an electrically driven cooling fan, comprisingin combination a resistor the value of which is dependent in use on thetemperature of an apparatus cooled by the fan, a relay winding having acontact which when the winding is de-energised serves to operate thefan, an amplifier coupled to said resistor and responsive to the valueof said resistor, said amplifier having an output decreasing with risein temperature of the apparatus whereby when the temperature of theapparatus falls to a predetermined level the amplifier output energisessaid relay winding, and resistance means which is coupled to theamplifier by way of the relay contact when the relay winding isenergised, the resistance means serving when the relay is energised to.reduce the current flowing in the relay winding by an amountinsufficient to cause the relay winding to become de-energised, soreducing the required increase in the temperature of the apparatusbefore the relay winding is again de-energised to cause the fan tooperate.

1. A control circuit for an electrically driven cooling fan, comprisingin combination a resistor the value of which is dependent in use on thetemperature of an apparatus cooled by the fan, a relay winding having acontact which when the winding is de-energised serves to operate thefan, an amplifier coupled to said resistor and responsive to the valueof said resistor, said amplifier having an output decreasing with risein temperature of the apparatus whereby when the temperature of theapparatus falls to a predetermined level the amplifier output energisessaid relay winding, and resistance means which is coupled to theamplifier by way of the relay contact when the relay winding isenergised, the resistance means serving when the relay is energised toreduce the current flowing in the relay winding by an amountinsufficient to cause the relay winding to become deenergised, soreducing the required increase in the temperature of the apparatusbefore the relay winding is again de-energised to cause the fan tooperate.